“This is an interesting paper, but there are several points to bear in mind.

“First, the researchers’ suggestion that reducing air pollution could reduce crime makes sense only if the increased crime rate that they observed at time of high pollution is actually caused by the pollution. The observational design of the study makes it impossible to be sure about that. Maybe there is some other factor that causes air pollution to be high on some days, and completely independently causes the crime rate to increase. If that’s the case, then reducing the pollution would have no effect on crime. The researchers adjusted statistically for some factors that might work in that way, but they could not allow for factors on which they have no data. It’s also possible that the cause might work in the other direction – for some reason, perhaps higher crime rates make air pollution get worse. The researchers do explain that these problems with cause might exist, but claim to have got round them by using an additional approach, taking wind direction into account as what’s called an instrumental variable. For that to be valid, as the researchers make clear, one has to be confident that, after allowing for variables such as other aspects of the weather, the only way that the wind direction could affect crime rates is through its effect on air pollution levels. That’s quite a strong assumption, and one that can’t be tested directly with the data that they have. I’m not convinced that it is true, and if it isn’t, then all the doubts about whether changes in air pollution actually cause changes in the crime rate do still remain. If we can’t be reasonably confident that this is really cause and effect, then the researchers’ discussions of how the causation might work must remain entirely speculative. Indeed in my view some of them appear pretty speculative anyway.

“Another point to make is that this is a paper in a discussion paper series, published by the institute with which the authors are associated, and I believe it has not yet been subject to scientific peer review. A proper peer review, which would go much further into the details than I have had time to do, might or might not find more serious issues with these research findings.”

* ‘Discussion Paper Series: Crime is in the Air: The Contemporaneous Relationship between Air Pollution and Crime’ by Malvina Bondy et al. will be published by IZA Institute of Labor Economics on Thursday 26 April 2018.

Declared interests

Prof Kevin McConway: “Kevin McConway was the lead author of a chapter on Measurement and Communication of Health Risks from Pollution for the 2017 Annual Report of the Chief Medical Officer ‘Health Impacts of All Pollution: what do we know?’. He is a member of the Science Media Centre’s Advisory Committee.”